Polycarbonates are well known polymers which have good property profiles, particularly with respect to impact resistance, electrical properties, dimensional rigidity and the like. These polymers are generally linear, but can be made with branched sites to enhance their properties in specific ways. Low levels of branching are generally incorporated into the resin by co-polymerizing into the polymer backbone a tri or higher functional reagent to yield a thermoplastic polycarbonate resin with enhanced rheological properties and melt strength which make it particularly suitable for such types of polymer processing procedures as the blow molding of large, hollow containers and the extrusion of complex profile forms.
Sufficiently higher levels of branching sites in the resin will cause resin chains actually to join to each other to form partially or fully crosslinked resin networks which will no longer be thermoplastic in nature and which are expected to exhibit enhancements, over corresponding linear resins, in physical properties and/or in their resistance to abusive conditions, such as exposure to organic solvents. A wide variety of means have been employed to produce crosslinking in polycarbonate resin. These generally involve the incorporation of a suitably reactive chemical group either into the resin chain at its time of manufacture or as an additive to the resin after manufacture, or both. These reactive groups and the reactions they undergo are generally dissimilar from those characteristic of polycarbonate resin itself and are therefore prone to have detrimental side effects on the physical and/or chemical properties of the polymer. The conventional test used to judge the success of these means for crosslinking is to observe the formation of gels due to the crosslinked material when a resin sample is mixed with a solvent, such as methylene chloride, in which normal linear polycarbonate resin is highly soluble.
A new method has been discovered to prepare branched or crosslinked polycarbonate resin. This approach involves incorporating a multifunctional comonomer of better than two reactive groups into cyclic bisphenol carbonate oligomers. The thus prepared cyclic oligomers are then reacted at elevated temperature with catalysis to yield high molecular weight polycarbonate resin. Generally the polymerization occurs under melt conditions. This reaction is thought to proceed by a multi-step ring opening addition mechanism. During this polymerization the functional groups of the multifunctional comonomer are available for building branches and/or for crosslinking one polycarbonate chain to another polycarbonate chain.
This new method to prepare branched or crosslinked polycarbonate resin is an improvement over previous methods in that the resin is initially low in molecular weight and thus has low viscosity and is easily processed into its desired forms. It is then converted under convenient reaction conditions to high viscosity branched resin or to cross-linked resin. This is accomplished by incorporating into the resin multifunctional comonomers with chemical groups with similar structure and reactivity to the repeat units of the resin so that the possibility of detrimental side effects on resin properties are minimized.